This invention relates to methods and apparatus for determining the characteristics of material surrounding a well bore and, more particularly, relates to methods and apparatus for radio frequency dielectric induction well logging wherein the formation resistivity (or conductivity) and dielectric constant are uniquely determinable by means of the in situ measurements taken in a well bore hole.
For many years, it has been conventional practice to log the electrical characteristics of earth formations in the vicinity of a well bore hole in order to determine the location of an oil bearing strata. This has been made possible by the use of electrical resistivity logs in wells using highly conductive (low resistivity) drilling fluid and by the use of induction well logs in wells drilled with oil base drilling muds or drilling fluids having higher resistivity. In conventional resistivity logs, a current emitting electrode (or electrode array) arranged for focusing the emitted current is used to emit either direct current or very low frequency (such as 60 hertz) alternating current into the earth formations surrounding the well borehole by means of contacting electrode. These currents traverse a section of the earth formations and are detected at a current return electrode situated at spaced distance from the current emitting electrode or electrode array. The magnitude of the detected currents can then provide an indication of the resistivity of the earth formation surrounding the well bore. In some cases, current electrodes are used in conjunction with measuring potential electrodes to determine the formation resistivity.
In electrical induction logging it has been conventional practice in the past to provide a well logging sonde having a transmitting coil (or array of coils) disposed thereon and a receiver coil (or array of coils) at a spaced distance therefrom. Generally, a high frequency alternating current is passed through the transmitter coil (normally at a frequency of approximately 20 kilohertz). The resulting electric fields produced from this high frequency alternating current in earth formations surrounding the well bore are detected at the spaced receiver coil by sensing the induced current or voltage in the receiver coil.
In both of these types of prior art resistivity (or conductivity) logging system, the usefulness of the system arises from the fact that earth formations having pore spaces therein filled with hydrocarbon molecules exhibit a higher resistivity than those earth formations having the pore spaces therein filled with either salt water or some other conducting fluid.
Various problems have arisen in the interpretation of either conventional induction logging records or resistivity logging records of wells in area where fresh water (relatively nonconductive, such as less than 10,000 parts per million sodium chloride) are encountered. Such fresh water bearing sands or earth formations exhibit high resistivity (or low conductivity) much the same as those encountered in hydrocarbon bearing formations. In these cases, it is difficult if not impossible to distinguish on the basis of the electric or induction well logging data alone, whether an earth formation which is a prospective producing zone contains fresh water or hydrocarbon. It would, therefore, be very beneficial to provide a well logging system which on the basis of a single measuring pass of some of the physical characteristics of the earth formation in the vicinity of the well borehole, could distinguish between fresh water bearing and hydrocarbon bearing earth formations.
Accordingly, it is an object of the present invention to provide a well logging system which is capable of distinguishing fresh water bearing earth formations from hydrocarbon bearing earth formations in the vicinity of the well borehole on the basis of electrical measurements.
Another object of the invention is to provide an electrical measurement well logging system which is capable of measuring simultaneously quantities relating to the conductivity and dielectric constant of earth formations in the vicinity of a well borehole.
Another object of the present invention is to provide an induction well logging system which operates at frequencies such that the dielectric properties of the media surrounding the well borehole influence the measurement together with the conductivity or resistivity characteristics of the material surrounding the well bore.